1. Field of the Invention
The present invention relates to an improved technique for embodying a radiation detector of very high performance that can be used for detecting in position ionizing radiations such as charged particles, photons, X-rays and neutrons.
2. Brief Description of the Prior Art
Radiation detectors exploiting the process of ionization and charge multiplication in gases have been in use with continued improvements since hundred years. Methods for obtaining large "stable" proportional gains in gaseous detectors are a continuing subject of investigation in the detectors community.
Several years ago, G. CHARPAK and F. SAULI introduced the multistep chamber, thereafter designated as MSC, as a way to overcome on limitations of gain in parallel plate and multiwire proportional chambers, thereafter designated as MWPC.
In MSC chambers, two parallel grid electrodes mounted in the drift region of a conventional gas detector and operated as parallel plate multipliers allow to preamplify drifting electrons and transfer them into the main detection element. Operated with a photosensitive gas mixture, the MSC chamber allows to reach gains large enough for single photodetection in ring-imaging CHERENKOV detectors, thereafter designated as RICH. For more details with respect to MSC chambers and RICH chambers, we refer to the following publications:
G. CHARPAK and F. SAULI, Physics Letters, vol.78B, 1978, p.523, and PA1 M. ADAMS and al., Nuclear Instrumentation Methods, 217, 1983, 237.
More recently, G. CHARPAK and Y. GIOMATARIS have developed an improved radiation detector device thereafter designated as MICROMEGAS which is a high gain gas detector using as multiplying element a narrow gap parallel plate avalanche chamber.
In a general point of view, such a detector consists of a gap in the range 50 to 100 .mu.m which is realized by stretching a thin metal micromesh electrode parallel to a read-out plane. G. CHARPAK and Y. GIOMATARIS have demonstrated very high gain and rate capabilities which are understood to result from the special properties of electrode avalanches in very high electric fields. For more details concerning the MICROMEGAS detector, we refer to the publication edited by Y. GIOMATARIS, P. REBOUGEARD, J. P. ROBERT and G. CHARPAK in Nuclear Instruments Methods, A376, 1996, 29.
The major point of inconvenience of both described detectors lies in the necessity of stretching and maintaining parallel meshes with very good accuracy. The presence of strong electrostatic attraction forces adds to the problem particularly for large size of the detectors. To overcome this drawback, heavy support frames are required and in the case of the MICROMEGAS detector the introduction in the gap of closely spaced insulating lines or pins with the ensuing complication of assembly and loss of efficiency is necessary.
Another radiation detector device was recently developed and proposed by F. BARTOL and al. Journal of Physics III 6 (1996), 337.
This detector device, thereafter designated as CAT, for Compteur a trous, substantially consists of a matrix of holes which are drilled through a cathode foil. The insertion of an insulating sheet between cathode and buried anodes allows thus to guaranty a good gap uniformity and to obtain high gains.